title: Association of cytochrome b5 and cytochrome P-450 reductase with cytochrome P-450 in the membrane of reconstituted vesicles.
abstract: A protein-protein association of cytochrome P-450 LM2 with NADPH-cytochrome P-450 reductase, with cytochrome b5, and with both proteins was demonstrated in reconstituted phospholipid vesicles by magnetic circular dichroism difference spectra. A 23% decrease in the absolute intensity of the Soret band of the magnetic CD spectrum of cytochrome P-450 was observed when it was reconstituted with reductase. A difference spectrum corresponding to a 7% decrease in absolute intensity was obtained when cytochrome b5 was incorporated into vesicles that already contained cytochrome P-450 and cytochrome P-450 reductase compared to a decrease of 13% in absolute intensity when cytochrome b5 was incorporated into vesicles that contained only cytochrome P-450. The use of the magnetic circular dichroism confirmed that protein-protein associations that have been detected by absorption spectroscopy between purified and detergent-solubilized proteins also exist in membranes. High ionic strength was shown to interrupt direct electron flow from cytochrome P-450 reductase to cytochrome P-450 but not the electron flow from reductase through cytochrome b5 to cytochrome P-450. Upon incorporation of cytochrome b5 into cytochrome P-450- and cytochrome P-450 reductase-containing vesicles, an increase of benzphetamine N-demethylation activity was observed. The magnitude of this increase was numerically identical to the residual activity of the reconstituted vesicles measured in the presence of 0.3 M KCl. It is concluded that there is a requirement for at least one charge pairing for electron transfer from reductase to cytochrome P-450. These observations are combined in a proposed mechanism of coupled reversible association reactions in the membrane.
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism*; D015786:Cytochromes b5; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism*; D010634:Phenobarbital / Q000494:pharmacology; D011817:Rabbits; D013053:Spectrophotometry
chemical_list: D003580:Cytochromes; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D009251:NADPH-Ferrihemoprotein Reductase; D010634:Phenobarbital